Mössbauer study of CO-precipitated Fischer-Tropsch iron catalysts

Mössbauer spectroscopy studies of precipitated Fischer-Tropsch (FT) iron catalysts, viz. 100 Fe/5 Cu/4.2 K/x SiO₂, wherex=0,8, 16, 24, 25, 40, or 100, have shown that reduction of the oxide precursor in CO gives rise to -carbide Fe₅C₂ whose amount decreases with an increase of SiO₂ content. The -carbide is converted into magnetite Fe₃O₄ while catalyzing the FT synthesis reaction. A correlation between FT activity and the content of -carbide in the catalysts was found, which indicated that -carbide is active for FT synthesis reaction.     

Application of Mössbauer Spectroscopy to the Carbon Oxides Hydrogenation Reactions

Iron-based catalysts have favorable activity and selectivity properties for the CO and CO₂ hydrogenation reactions. Several Fe phases (oxides and carbides) can be present in these catalysts. The interaction of Fe with the other components of the catalyst (support, promoters) can affect the ease of reduction and also its transformation during the reactions. In this work, the relationship between catalytic behavior in the CO and CO₂ hydrogenation reactions and the Fe phase composition of fresh and reacted catalysts was studied. Two types of catalysts were tested: a laterite and the other one made of iron supported on alumina, both unpromoted and promoted with K and Mn. Only those Fe species which can be reduced-carburized, by means of a pretreatment or by an in situ transformation under the reaction, seem to be able to perform the CO or CO₂ hydrogenation. The reoxidation of the Fe carbide to magnetite was not associated to deactivation. The selectivity seems to be more affected by Fe species difficult to reduce than by magnetite produced by reoxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Examining the redox and formate mechanisms for water–gas shift reaction on Au/CeO2 using density functional theory

We perform DFT calculations to investigate the redox and formate mechanisms of water–gas-shift (WGS) reaction on Au/CeO₂ catalysts. In the redox mechanism, we analyze all the key elementary steps and find that the OH cleavage is the key step. Three possible pathways of OH cleavage are calculated: (1) OH_ad^″+*^′→H_ad^′+O_ad^″; (2) H_ad^′+OH_ad^″→H₂(g)+O_ad^″+*^′; and (3) OH_ad^″+OH_ad^″→2O_ad^″+H₂(g) (*′: the free adsorption sites on the oxides; ad′: adsorption on the metal; ad″: adsorption on the oxide, respectively). In the formate mechanism, we identify all the possible pathways for the formation and decomposition of surface formates in the WGS reaction. It is found that there is a shortcoming in the redox and formate mechanisms which is related to surface oxygen reproduction. Four possible pathways for producing surface oxygen are studied, and all the barriers of the four pathways are more than 1 eV. Our results suggest that the processes to reproduce surface oxygen in the reaction circle are not kinetically easy.     

Alumina contribution to CO oxidation: A TPR and IR study

The alumina contribution to CO oxidation in the absence of O₂ on metal oxide supported catalysts was investigated by CO TPR and in-situ FTIR and DRIFT studies up to 800 °C. These tests were performed on two Al₂O₃ supported catalysts (1 wt.% Pt/La/γ-Al₂O₃ and 8 wt.% Cu/γ-Al₂O₃) and on two corresponding alumina supports (5 wt.% La₂O₃ stabilised γ-Al₂O₃ and high mechanical resistant spherical γ-Al₂O₃ particles). The quantitative determination of CO consumption and CO₂ and H₂ formation on the alumina supports was in agreement with a WGS reaction occurring between surface OH and CO with a predominantly 2:1 stoichiometry. In the CO TPR of metal oxide supported catalysts, in addition to the reduction of the metal, a WGS reaction took place with enhanced kinetics. This enhancement was the result of a CO spillover phenomenon from the metal to alumina hydroxyls. This phenomenon significantly affected the evaluation of the reduction degree of the supported metal and could not be neglected in the subsequent calculations.     

Photocatalytic activity of multielement doped TiO2 in the degradation of congo red

TiO₂ although considered a promising photocatalyst for the degradation of aqueous pollutants, it suffers from poor absorption in the visible region and hence requires ultraviolet (UV) light for activation. To make TiO₂ a visible active photocatalyst, multielement (C, N, B, and F) doping has been done. The synthesised CNBF/TiO₂ catalysts were calcined at different temperatures and characterized by XRD, BET surface area, UV DRS, XPS, HRSEM-EDAX, and TEM techniques. These catalysts found to show less band gap values when compared to bare TiO₂. These catalysts were tested for their catalytic activity towards the degradation of a textile dye - congo red (CR) under different reaction conditions. It was found that the photocatalytic activity was dependent on both doping of multielement and the calcination temperature of CNBF/TiO₂. The co-doped catalysts which were calcined at 400 °C and 600 °C (100% intensity in anatase phase) were found to be the best catalysts (100% decolourisation of CR in 21/2 h and 2 h respectively). TOC analysis carried out for the samples at the reaction time of 5 h showed very high percentage (83%) degradation of CR over CNBF/TiO₂ catalysts calcined at 600 °C when compared to the other catalysts calcined at different temperatures. CNBF/TiO₂ (1000 °C) showed very less photocatalytic activity due to the formation of rutile phase.     

Studies of the activity of catalysts based on heteropolyacids

The catalytic activity of samples such as PPy(H₄SiW₁₂O₄₀), PPy(H₅PMo₁₀V₂O₄₀), PPy(H₂Fe(III)PMo₁₀V₂O₄₀), PPy(H₃Cu(II)PMo₁₀V₂O₄₀) has been examined in two different test reactions. The acid-base and oxidation-reduction properties were studied using the conversion of isopropyl alcohol to propene and acetone. Redox ability of catalysts was examined in the reaction of oxidation of allyl alcohol to glycidol. It was found that the activity of catalysts in the oxidation of allyl alcohol increases as the oxidation properties determined from the conversion of isopropyl alcohol increase. It was also observed that stronger oxidation-reduction properties of the catalyst result in a high rate of the consecutive reaction of glycidol to 3-hydroxypropanone.The phase composition of catalysts was determined by means of X-ray diffraction (XRD).     

Insight into the reaction mechanism of ethanol steam reforming catalysed by Co–Mo6S8

The production of hydrogen via steam reforming of ethanol (SRE) is favourable for the use of hydrogen as an alternative fuel. Co–Mo₆S₈ possesses high activity and stability for SRE to sustainably produce hydrogen. The competition among reaction pathways related to C–H, O–H, C–C, C–O cleavage and H₂ formation was studied. The adsorption and reaction of related intermediates in the ESR reaction pathway are described. The results indicated that the most feasible route for the decomposition of ethanol catalysed by Co–Mo₆S₈ is CH₃CH₂OH*→CH₃CH₂O*→CH₃CHO*→CH₂CHO*→CHCHO*→CHCO*→CH*+CO*. The CH* can be decomposed into C*+H*, and CO* can be oxidised via the redox mechanism of the water gas shift (WGS) reaction. Thus the final products are CO₂ and H₂. The present result may help people to design an SRE catalyst, which has the ability to break C–C to form CO and H₂, then CO react with H₂O in the WGS reaction generating CO₂ and H₂.     

Mössbauer study of the evolution of a laterite iron mineral based catalyst: Effect of the activation treatment

The syngas reaction has been studied using a laterite iron mineral, promoted with K and Mn. In situ activation under syngas, as well as pre-treatment with H₂ followed by CO under mild and more severe conditions were tested. These activation procedures led to different iron phase compositions and to different catalytic selectivities. The C₂-C₄/CH₄ ratio was significantly lower for those catalysts which after reaction showed the presence of hexagonal carbide and magnetite compared to the solid, which showed the highest proportion of Hägg carbide.     

Origin and activity of oxidized gold in water-gas-shift catalysis

As a promising route for large-scale H2 production, the water-gas-shift reaction (WGS, CO + H(2)O-->CO(2) + H(2)) on ceria-supported Au catalysts is of enormous potential in efforts to move towards a hydrogen economy. Recent research suggests that this reaction is in fact catalyzed by Au cations instead of the conventionally regarded metallic Au particles. Here density functional theory calculations demonstrate that the presence of empty localized nonbonding f states in CeO2 permits the oxidation of Au, enabling subsequent CO adsorption. A feasible reaction pathway leading to H2 production is identified, and it is concluded that four to six atom Au clusters at the O-vacancy sites of ceria catalyze the WGS reaction.     

Halobenzene activation by heterofullerenes: computational investigation of oxidative addition activity

Potential of Pd‐ and Ni‐substituted fullerenes for oxidative addition of halobenzenes was investigated using density functional theory. The metal centers in the catalysts were found to be the potential reaction sites. Adsorption of halobenzenes was mildly exergonic over both the compounds. Activation of all halobenzenes was observed over both the compounds. Oxidative addition of C₆H₅I was found to be the least energy intensive process with a free energy requirement that was 3 times smaller than that for C₆H₅F over C₅₉Pd. Activities of both the catalysts were found to be comparable with the end products differing in the coordination of phenyl ring with the heterofullerenes.     

Preparation of improved catalytic materials for water purification

The aim of presented paper was to study preparation of catalytic materials for water purification. Iron oxide (Fe₃O₄) samples supported on activated carbon were prepared by wet impregnation method and low temperature heating in an inert atmosphere. The as-prepared, activated and samples after catalytic test were characterized by Mössbauer spectroscopy and X-ray diffraction. The obtained X-ray diffraction patterns of prepared samples show broad and low-intensity peaks of magnetite phase and the characteristic peaks of the activated carbon. The average crystallite size of magnetite particles was calculated below 20 nm. The registered Mössbauer spectra of prepared materials show a superposition of doublet lines or doublet and sextet components. The calculated hyperfine parameters after spectra evaluation reveal the presence of magnetite phase with nanosize particles. Relaxation phenomena were registered in both cases, i.e. superparamagnetism or collective magnetic excitation behavior, respectively. Low temperature Mössbauer spectra confirm this observation. Application of materials as photo-Fenton catalysts for organic pollutions degradation was studied. It was obtained high adsorption degree of dye, extremely high reaction rate and fast dye degradation. Photocatalytic behaviour of a more active sample was enhanced using mechanochemical activation (MCA). The nanometric size and high dispersion of photocatalyst particles influence both the adsorption and degradation mechanism of reaction. The results showed that all studied photocatalysts effectively decompose the organic pollutants under UV light irradiation. Partial oxidation of samples after catalytic tests was registered. Combination of magnetic particles with high photocatalytic activity meets both the requirements of photocatalytic degradation of water contaminants and that of recovery for cyclic utilization of material.     

Mössbauer spectroscopy for the characterization of soils used as catalysts

We have recently reported the use of volcanic ash soil as an iron supported catalyst for the water gas shift reaction, taking into account that one of the characteristics of Chilean soils is their large specific surface area and high iron oxide contents. Here we report the characterization of the soil material which was used as catalysts by means of Mössbauer spectroscopy at room and low temperature. The spectra were dominated by a broad central doublet, which was analyzed into two components attributable to paramagnetic ferric iron. They also show two low intensity magnetically splitted sites, withIS andH _eff which may be assigned to magnetite or a similar substituted ferrite. No hematite or other common soil iron oxides seem to be present on these samples, but the presence of low crystallinity ferrihydrite may be asociated with the, central doublets.     

Monitoring by Mössbauer spectroscopy the thermal reduction of hematite into magnetite: the surface effect and charge disproportionality in iron oxide nanoparticles

Nanoparticles of magnetite Fe₃O₄ were synthesized by thermal reduction of hematite α-Fe₂O₃ powder in the presence of high boiling point solvent. The structural transformations and magnetic properties of the obtained nanoparticles were investigated by the ⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction, and magnetic measurements. The content of hematite and magnetite phases was evaluated at each step of the chemical and thermal treatment of the product. An increase of saturation magnetization with the reaction time correlates with an increase of concentration of magnetite in the samples. The electron hoping between Fe^2?+? and Fe^3?+? ions in the octahedral sites of the magnetite nanoparticles and Verwey phase transition were investigated. It was established that not all iron ions in the octahedral sites participated in electron hoping Fe^2?+????Fe^3?+? above the Verwey temperature T _V, and the charge distribution could be expressed as $\big( {{\rm Fe}^{3+}}\big)_{{\rm tet}} \big[ {{\rm Fe}_{1.85}^{2.5+} {\rm Fe}_{0.15}^{3+} }\big]_{{\rm oct}} {\rm O}_4$ .     

Effect of Calcination Temperature on La-Modified Al2O3 Catalysts for Vapor Phase Hydrofluorination of Acetylene to Vinyl Fluoride

利用沉积沉淀法制备出了La改性Al₂O₃催化剂,研究不同焙烧温度对La改性Al₂O₃催化剂用于乙炔气相氢氟化合成氟乙烯反应性能的影响．利用NH₃-TPD、Pyridine-FTIR、XRD和Raman等技术对不同温度焙烧的催化剂进行表征,发现焙烧过程能改变催化剂结构的同时也能调变催化剂表面的酸量．经400 °C焙烧的催化剂显示出最高的乙炔转化率(94.6%)、最高的氟乙烯选择性(83.4%)和较低的积炭选择性(0.72%)．催化剂的高活性与其表面的高酸量有关,同时积炭的选择性也与其表面的酸中心数量有关．     

Covalent immobilization of invertase on PAMAM-dendrimer modified superparamagnetic iron oxide nanoparticles

In this study, polyamidoamine (PAMAM) dendrimer was synthesized on the surface of superparamagnetite nanoparticles to enhance invertase immobilization. The amount of immobilized enzyme on the surface-hyperbranched magnetite nanoparticle was up to 2.5 times (i.e., 250%) as much as that of magnetite nanoparticle modified with only amino silane. Maximum reaction rate (V _max) and Michaelis–Menten constant (K _m) were determined for the free and immobilized enzymes. Various characteristics of immobilized invertase such as; the temperature activity, thermal stability, operational stability, and storage stability were evaluated and results revealed that stability of the enzyme is improved upon immobilization.     

The role of copper species on Cu/γ-Al2O3 catalysts for NH3-SCO reaction

UV-vis spectra, XRD, H₂-TPR, TEM and ESR were used to characterize a series of Cu/γ-Al₂O₃ catalysts, which were prepared by incipient wetness impregnation using copper nitrate, copper acetate or copper sulfate as precursors, to study the role of Cu species on Cu/γ-Al₂O₃ catalysts for NH₃-SCO reaction. It was found that the mixture of CuO phase and CuAl₂O₄ phase formed on various Cu/γ-Al₂O₃ catalysts, and the Cu species and dispersion had significant influence on the Cu/γ-Al₂O₃ activity. Highly dispersed CuO phase on the support would be related with its high activity for the NH₃-SCO reaction.     

Mössbauer study of a Fe3O4/PMMA nanocomposite synthesized by sonochemistry

Magnetite nanoparticles of 10 nm average size were synthesized by ultrasonic waves from the chemical reaction and precipitation of ferrous and ferric iron chloride (FeCl₃ · 6H₂O y FeCl₂ · 4H₂O) in a basic medium. The formation and the incorporation of the magnetite in PMMA were followed by XRD and Mössbauer Spectroscopy. These magnetite nanoparticles were subsequently incorporated into the polymer by ultrasonic waves in order to obtain the final sample of 5 % weight Fe₃O₄ into the polymethylmethacrylate (PMMA). Both samples Fe₃O₄ nanoparticles and 5 % Fe₃O₄/PMMA nanocomposite, were studied by Mössbauer spectroscopy in the temperature range of 300 K–77 K. In the case of room temperature, the Mössbauer spectrum of the Fe₃O₄ nanoparticles sample was fitted with two magnetic histograms, one corresponding to the tetrahedral sites (Fe^3?+?) and the other to the octahedral sites (Fe^3?+? and Fe^2?+?), while the 5 % Fe₃O₄/PMMA sample was fitted with two histograms as before and a singlet subspectrum related to a superparamagnetic behavior, caused by the dispersion of the nanoparticles into the polymer. The 77 K Mössabuer spectra for both samples were fitted with five magnetic subspectra similar to the bulk magnetite and for the 5 % Fe₃O₄/PMMA sample it was needed to add also a superparamagnetic singlet. Additionally, a study of the Verwey transition has been done and it was observed a different behavior compared with that of bulk magnetite.     

Thermally Evaporated Ag–Au Bimetallic Catalysts for Efficient Electrochemical CO2 Reduction

CO₂ reduction reaction (CO₂RR) has indispensable significance for carbon recycling and renewable energy production. As typical electrochemical catalysts, Au and Ag show relatively high reaction activity and selectivity in CO₂RR. In this study, a series of Ag–Au bimetallic catalysts are designed and synthesized through the thermal evaporation method for efficient yet massive production of electrochemical catalysts. The Ag–Au catalysts show significantly enhanced activity and selectivity in CO₂RR, which is mainly attributed to the increased grain boundaries with well-dispersed single Ag atoms. After the optimization, Au20Ag10 exhibits the best performance with a CO Faraday efficiency of 89% at −0.9 V (vs the reversible hydrogen electrode) with good stability.     

Immobilization of metalloporphyrins on P(4VP-co-St)/SiO2 by the quaternarization reaction

Poly(4vinylpyridine-co-styrene) (P(4VP-co-St)) was grafted on silica gel particles in the manner of “grafting from”, and the grafting particle P(4VP-co-St)/SiO₂ was gained. The chloromethylation reaction for the tetraphenylporphyrin (TPP) was performed using a chloromethylation reagent, 1,4-bis(chloromethyoxy)butane which was uncarcinogenic, and the tetra-chloromethylphenyl-porphyrin (TMCPP) was prepared. Then, the quaternization reaction between the benzyl chloride groups on TMCPP and pyridine groups of the grafted P(4VP-co-St) macromolecules occurred and the bonding of TMCPP on the particles P(4VP-co-St)/SiO₂ was realized, resulting in the functional composite-type particles TMCPP-P(4VP-co-St)/SiO₂. Subsequently, the metallation of the bonded particles TMCPP-P(4VP-co-St)/SiO₂ was carried out via the coordination reaction between TMCPP-P(4VP-co-St)/SiO₂ and metal salt, resulting in the supported metalloporphyrin (MP) catalysts MP-P(4VP-co-St)/SiO₂. The supported catalysts were characterized by UV-Vis spectra. The effects of various factors on the bonding process of TMCPP on P(4VP-co-St)/SiO₂ were studied in detail. In addition, the catalytic activity of the supported catalysts MP-P(4VP-co-St)/SiO₂ have been studied in oxidation process of ethyl benzene with molecular oxygen to acetophenone without the use of sacrificial co-reductant. The experimental results showed that the tetra-chloromethylphenyl-porphyrin (TMCPP) could be successfully bonded onto the P(4VP-co-St)/SiO₂ surfaces by means of the quaternization reaction between TMCPP and the pyridine groups on grafted P(4VP-co-St) macromolecules. The supported catalysts MP-P(4VP-co-St)/SiO₂ exhibited the fine catalytic activity. Moreover, the supported cobalt porphyrin was more active than the supported iron and manganese porphyrins.     

Morphological investigation of nanostructured CoMo catalysts

This work reports the morphological investigation of nanostructured sulfided CoMo catalysts by means of high-resolution transmission electron microscopy (HRTEM). The catalysts were supported on Ti-modified hexagonal mesoporous silica (HMS-Ti) and P-modified HMS-Ti (P/HMS-Ti) materials. The oxide precursors were characterized by specific surface area (S_BET), temperature-programmed reduction (TPR), diffuse reflectance infrared Fourier transform spectroscopy in the OH region (DRIFTS-OH) and X-ray photoelectron spectroscopy (XPS) in order to elucidate the influence of the impregnation sequence (successive vs. simultaneous) and the effect of P-incorporation into HMS-Ti material on the morphology of calcined CoMo catalysts. Both TPR and XPS measurements indicate that the catalysts prepared by successive impregnation possess well-dispersed MoO₃ and CoO phases, whereas their counterparts prepared by simultaneous impregnation additionally possess the CoMoO₄ phase. For all sulfided catalysts, the presence of MoS₂ phase with particle size in the range 3.3-4.4 nm was confirmed by HRTEM. Catalytic activity was evaluated in the reaction of hydrodesulfurization (HDS) of dibenzothiophene (DBT) carried out in a flow reactor at 593 K and hydrogen pressure of 5.5 MPa. P-incorporation into the HMS-Ti material led to an overall increase in HDS activity and the hydrogenation ability of the sulfided catalysts. All catalysts proved to be stable during 10 h time-on-stream (TOS) operation. The activity of sulfide catalysts in the target reaction depends linearly on the surface exposure of Co species in the oxide precursors, as determined by XPS, and on the morphology of the sulfide form of catalysts (surface density of MoS₂ particles and their sizes) as determined by HRTEM.